option to erase expandable disks

[Michal Suchanek]

The expandable disks are nice but there is no way to shrink the disk once it is expanded.

Deleting and recreating the disk image is quite annoying as it must be unregistered and re-registered with the VM.

There are two possibilities:

• erase the disk completely - possibly an option in the config of the VM or the disk image manager - this would be helpful for test machines

• discard zero sectors - this could be done as they are written - could be used to save space by erasing free space inside the guest OS

[Frank Mehnert]

Sectors containing only zeros can be discarded with

VBoxManage modifyvdi FILENAME compact

We don't do this during runtime as this would decrease the write performance.